1. Field of the Invention
The present invention relates to a process for producing a silicon single crystal and more particularly to a process for pulling up the silicon single crystal by the Czochralski method (referred to as a CZ method hereinafter).
2. Description of the Related Art
In growing a silicon single crystal by the CZ method, a dopant such as phosphorus (P) or boron (B) is added to a silicon melt in a crucible after a raw silicon polycrystal is melted and the silicon single crystal is grown after the concentration of the dopant in the melt is controlled. However, since the segregation coefficient of the dopant is less than one (the segregation coefficient of phosphorus is 0.35,the segregation coefficient of boron is 0.75), the concentration of the dopant in the melt increases during growth of the silicon single crystal. And the resistivity of the resulting silicon single crystal rod decreases from the head to the tail of the silicon single crystal rod. Since the usable length of the silicon single crystal rod must be so much reduced as an allowable range of a dispersion in the resistivity is narrow, a product weight to raw material weight ratio is reduced to reduce the yield of the silicon single crystal. In order to maintain constant the concentration of the dopant in the melt and thereby grow a silicon single crystal of a uniform resistivity dispersion in the direction of the growth of the silicon single crystal, there have been developed a method of continuously charging appropriate amounts of undoped silicon polycrystal during growth of the silicon single crystal (a melt-reduced continuous charge method) and a method of charging appropriate amounts of undoped silicon polycrystal and appropriate amounts of a dopant in response to a degree of growth of the silicon single crystal to continuously maintain constant the amount of the melt and the concentration of the dopant in the melt (a melt-constant continuous charge method).
Silicon polycrystal ingot or a block-shaped silicon polycrystal has been employed as the raw silicon polycrystal in both the melt-reduced and melt-constant continuous charge methods. Since simply soaking the silicon polycrystal ingot in the melt cannot control the amount of silicon polycrystal ingot fed to the melt, it has been proposed that a heater for melting the silicon polycrystal ingot is provided in a chamber for growth of single crystal or that a chamber is separated into a chamber for growth of single crystal and a chamber for melting raw silicon polycrystal and a quartz piping connects the chamber for growth of single crystal to the chamber for melting raw silicon polycrystal. However, the both cases are impractical since the systems of the cases are complicated and expensive.
On the other hand, since the weights of block-shaped silicon polycrystals are relatively heavy and a dispersion in the weights of block-shaped silicon polycrystals is high when the block-shaped silicon polycrystals are employed as the raw material, it is difficult to precisely control the amount of silicon polycrystal fed to the melt. In addition, since the weights of the block-shaped silicon polycrystals are heavy, a feed piping for the raw silicon polycrystal must be designed in view of a physical impact on the feed piping.
In order to overcome the above-described problems, it has been recently proposed that grains of silicon polycrystal produced by a reaction on a fluidized bed from a high purity silane or trichlorosilane are employed as the raw silicon polycrystal. This method requires no provision of a heater for melting the grains of silicon polycrystal and can precisely control the amount of the raw silicon polycrystal fed to the melt. In addition, a feeder for the grains of silicon polycrystal is easily designed.
However, the present inventors discovered that since the production of the grains of silicon polycrystal by the reaction on the fluidized bed contains a large amount of residual hydrogen in the grains of silicon polycrystal, when the grains of silicon polycrystal are dropped into the melt, the melt splashes during a continuous charge of the grains of silicon polycrystal. That is, since the grains of silicon polycrystal are instantly exposed to an ultra high temperature (melting point of silicon of 1420.degree. C.) when they dropped to the melt, the residual hydrogen in the grains of silicon polycrystal causes splashes of the melt. On the other hand, a dehydrogenation of the grains of silicon polycrystal by a high temperature heat treatment for preventing the splashing of the melt tends to make a growing silicon single crystal polycrystalline. It is supposed that since the dehydrogenated grains of silicon polycrystal require a long time until they are fully melted in the melt after the drops of the grains to the melt, the isolation effect of a separating wall (internal crucible) is relatively reduced and semimolten grains of silicon polycrystal rise to the surface of the melt in the internal crucible. That is, it is supposed that a probability that grains of silicon polycrystal in a solid state go around the bottom of the internal crucible and reach the silicon single crystal rod is increased. Thus, the silicon single crystal rod tends to be made polycrystalline.